Photoacoustic endoscopy (PAE) is an exciting new tool specifically for many endoscopic diagnosis. We present a miniature all-optical probe that enables forward-view PAE without using a scanner. The probe utilizes an imaging fiber bundle and a gradient-index (GRIN) objective lens for delivery, focusing, and scanning of excitation laser pulses. Due to the fixed spatial relationship between the entrance and the output of the imaging fiber bundle, the laser scanning can be implemented on the proximal end face, and coherently transferred to the distal end face. The output light from the imaging fiber bundle is then focused by the GRIN objective lens, which results in an enlarged field of view (FOV) and working distance. For acoustic detection, a fiber-optic Fabry-Perot sensor is integrated to detect the generated acoustic waves from a wide field. The outer diameter of the PAE probe is 2.4 mm. The forward-view endoscopic imaging is presented to show the imaging capability of the PAE probe. The results show that our PAE probe is capable of forward-view photoacoustic imaging with high resolution (7.8−10.4 μm) and wide FOV (>3.5 mm).
Photoacoustic (PA) imaging forms an image based on optical absorption contrasts with ultrasound (US) resolution. In contrast, US imaging is based on acoustic backscattering to provide structural information. In this study, we develop a miniature all-optical probe for high-resolution PA-US dual-modality imaging over a large imaging depth range. The probe employs three individual optical fibers (F1-F3) to achieve optical generation and detection of acoustic waves for both PA and US modalities. To offer wide-angle laser illumination, fiber F1 with a large numerical aperture (NA) is used for PA excitation. On the other hand, wide-angle US waves are generated by laser illumination on an optically absorbing composite film which is coated on the end face of fiber F2. Both the excited PA and backscattered US waves are detected by a Fabry-Pérot cavity on the tip of fiber F3 for wide-angle acoustic detection. The wide angular features of the three optical fibers make large-NA synthetic aperture focusing technique possible and thus high-resolution PA and US imaging. The probe diameter is less than 2 mm. Over a depth range of 4 mm, lateral resolutions of PA and US imaging are 104−154 μm and 64−112 μm, respectively, and axial resolutions of PA and US imaging are 72−117 μm and 31−67 μm, respectively. To show the imaging capability of the probe, phantom imaging with both PA and US contrasts is demonstrated. The results show that the probe has potential for endoscopic and intravascular imaging applications that require PA and US contrast with high resolution.
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